Metal fuels comprising metals and oxidizers have been used for many years to produce heat, and power for propulsion in a variety of applications ranging from rocket motors to heat sources for steam boilers used in torpedoes. The selection of a metal and oxidizer, and their physical configuration, varies with the application depending on the system mission, heat output requirements, permissible reaction products, available space and many other factors.
A major distinction in this regard has to do with whether the system is open or closed with respect to expulsion of the reaction products. If the system is open so that the reaction products can be exhausted to the external environment, considerations such as pressure buildup due to the evolution of gaseous reaction products is not an issue. In fact, as is the case for solid rocket motors, it may be essential to the performance of the system. In circumstances, however, where the system is closed with respect to the expulsion of reaction products, particularly gaseous reaction products, to the external environment such as in torpedo propulsion applications, the permissible combinations of metals and oxidizers and their physical configuration are limited because the reaction products must be contained within the propulsion system. Gaseous reaction products present a particular problem. Generally, metal fuel/oxidizer systems operate at relatively high temperatures. Evolution of gaseous reaction products, even in relatively small amounts, at the operating temperatures of these systems can produce undesirable operating pressures within these closed systems according to the Ideal Gas Law: EQU PV=nRT
Where P is the system pressure, T is the system operating temperature, n is the number of moles of the gaseous reaction product, R is a constant depending on the units used for the other variables, and V is the volume of the system.
The present invention is directed toward, but not limited to use in, applications where the system is closed and the evolution of gaseous reaction products during the oxidation of the metal fuel is undesirable.
While many combinations of metal fuels and oxidizers are possible, it has been noted previously that lithium metal can be used in combination with fluorocarbon polymer oxidizers in applications where the system must be closed with respect to its external environment. In particular, lithium is currently used as a metal fuel in closed-loop torpedo applications. Lithium has been utilized because of its relative commercial availability and its ability to chemically react with numerous oxidizers. Fluorocarbon polymers have been utilized as oxidizers principally because they provide large heats of reaction and because they generally do not produce significant amounts of gaseous reaction products.
One particular combination which has been suggested utilizes pellets or granules of lithium which have had their surfaces coated with a relatively thin layer of fluorocarbon polymer oxidizer. In this system the reaction between the lithium and fluorocarbon polymer is initiated by an explosive detonator or similar device. The resulting reaction between a portion of the lithium and the fluorocarbon polymer is used to produce sufficient heat to melt the remaining lithium so that it can be further reacted with an oxidizing medium such as gaseous sulfur hexafluoride to provide the heat necessary to drive a steam boiler for a torpedo. It should be particularly noted, that the reaction between the lithium and fluorocarbon polymer oxidizer is not the principal source of heat in this system. The lithium/fluorocarbon polymer reaction is only intended to generate sufficient heat to melt the lithium granules so that the molten lithium can be reacted with the main oxidizing medium, sulfur hexafluoride.
The lithium/fluorocarbon polymer combination has a significant disadvantage, however, in that for a plurality of pellets, it is possible to initiate a hypergolic oxidation reaction between the lithium an fluorocarbon polymer accidentally due to vibration, mechanical shock or other movement of the pellets with respect to one another. Such motion can cause adjacent pellets to rub against one another. This mechanical rubbing, because of the frictional forces involved, can produce heat sufficient to initiate the reaction between the lithium and fluorocarbon polymer coating. Accidental initiation of this oxidation reaction presents significant safety concerns related to handling of torpedoes which utilize these fuels.
It has been suggested that barrier layers of other polymeric materials could be placed either over the outer surface of the fluorocarbon polymer oxidizer, or between the lithium and the oxidizer layer to protect against frictional rubbing of adjacent pellets. The principal requirement set forth for such a barrier layer is that it be less reactive with the lithium fuel than is the fluorocarbon polymer oxidation layer and thereby, have a lesser probability of accidental hypergolic reaction with the metal fuel. In such a system, the fuel pellets are allowed to move with respect to one another, and the added polymer layer is expected to reduce the friction between adjacent pellets and thereby lessen the probability of accidental hypergolic reaction of the lithium and fluorocarbon polymers.
A significant problem exists with this method, however, in that polymers suggested for use as the barrier layer, including polyparaxylene or dichloropolyparaxylene, contain elements which produce gaseous reaction products, such as hydrogen or hydrogen containing compounds, when the lithium is ultimately oxidized by the fluorocarbon polymer oxidizer. The evolution of gaseous reaction products can lead to pressurization of the system in which the fuel is located, and result in catastrophic failure of a closed-loop propulsion system.
At the operating temperatures of systems utilizing lithium fuels, which can be upwards of 1,000 degrees centigrade, the evolution of gaseous reaction products and resulting pressurization of the system can impose very significant design constraints, such as the need for pressure vessel containers for the fuel. Therefore, it is very desirable to develop other combinations which prevent the hypergolic reaction of lithium, as well as other metal fuels, and oxidizer coatings.